It is a common desire to enhance images acquired from imaging devices such as medical diagnostic (radiographic) imaging devices. Such enhancement may involve amplifying detail or local contrast in regions of interest, increasing or decreasing global contrast and sharpening edges. Enhancement algorithms, however, typically have the undesirable effect of enhancing and amplifying noise. It is therefore beneficial to provide a system for enhancing images while suppressing noise amplification or even reducing noise.
U.S. Pat. No. 5,461,655, by P. Vuylsteke and P. Dewaele, entitled "Method and Apparatus For Noise Reduction," describes a multiresolution noise reduction method that includes dynamic range compression. The disclosed method has the following features.
Only noise suppression and contrast modification are addressed. There is no mention of how to increase the amplitude of detail images while suppressing noise in order to yield sharper images. It is desirable to enhance images while controlling noise. PA1 The multiresolution representation involves a hierarchy of difference images, where the image size decreases at coarser scales. It is desirable to minimize the cost and time involved in subsampling and interpolating. PA1 A means for including anatomical information in the processing is not provided, whereas it is desirable to consider anatomical information when determining the amount of enhancement to perform. PA1 The noise estimation involves deriving a single quantity for noise in a detail image. Because noise varies across regions of the image, and with respect to code value, it is desirable to provide more reduction which adapts to spatially or code value varying noise. PA1 Determining local image content is a separate step from noise estimation. It is desirable to consider local image content in the noise estimation. PA1 Dynamic range compression occurs in serial with noise suppression. It is desirable to modify dynamic range, enhance detail and suppressed noise in parallel, allowing for more efficient operation.
The following patents are also relevant to the present invention.
U.S. Pat. No. 5,363,209, by R. Eschbach and W. A. Fuss, titled "Image-Dependent Sharpness Enhancement".
U.S. Pat. No. 4,941,190, by T. Joyce, titled "Method and System For Enhancement of a Digitized Image".
U.S. Pat. No. 4,827,528, by A. Macovski, titled "Error-Minimizing Noise Reduction System". In the disclosed method however, the low frequency image is unmodified.
A paper titled "Automatic Anatomically Selective Image Enhancement in Digital Chest Radiography," published in IEEE Transactions on Medical Imaging, volume 8, number 2, June 1989, by Sezan, Tekalp and Schaetzing describes a method of deriving an anatomically based lookup table.
Projection radiographic images can be acquired by several methods, including computed radiography (such as the KODAK DIGITAL SCIENCE Computed Radiography System 400) and direct digital radiography (such as the TFT flat panel systems now under development). In any case, the resulting digital code-value data represents a mathematical transformation of the x-ray exposure incident upon the imaging detector (for example the logarithm of the exposure). An appropriate look-up table is needed to select a limited range of these code values into code values that can be used to generate a film image by means of a laser printer or to view the image on a display device (such as a workstation CRT). The best look-up table is often a compromise between one with adequate latitude that renders the required range of exposures and one that gives adequate contrast for visualizing features within the image.
None of the techniques disclosed in these patents provide a satisfactory solution to image enhancement in medical image processing.